Method for heat treating steel wire rod

ABSTRACT

Patenting hot-rolled rod with alternate quenching and heat compensation stages before transformation; the quenching stations decrease logarithmically and the compensation stages increase logarithmically; the surface of the rod reaches is cooled down to below about 400*C, and the rod begins to transform in the pearlite zone before full temperature equalisation, the rod being reeled when a substantial peripheral zone of the rod has been predominantly transformed to pearlite.

O United States Patent 91 [111 3,735,966

Hoifmann 1 May 29, 1973 4 METHOD FOR HEAT TREATING [56] References CitedSTEEL WIRE ROD UNITED STATES PATENTS [75] Bend Emma, Duesseldmf,2,756,169 7/1956 Carson et al. ..14s/134 many 2,994,323 8/1961 Lewis..266/3 R x [73] Assignee; schloeman Akfi u h ft B 3,011,928 12/1961Kapec et al ..l48/l56 seldorf, Germany Primary Examiner-Gerald A. Dost 2Fl d: 6 [2 1 l 6 June 1971 Attorneyl-lolman & Stern 21 Appl. No.2150,806

[57] ABSTRACT Related US. Application Data Patenting hot-rolled rod withalternate quenching and [63] cmtmuamn of July 8, 1968' heat compensationstages before transformation; the abandoned quenching stations decreaselogarithmically and the [52] U S Cl 266/3 R 148/134 148/145 compensationstages increase logarithmically; the sur- 148/156 face of the rodreaches is cooled down to below about 51 Int. Cl. ..C2ld 9/52 C and therod begins to transform in the Pearlite [58] Field of Search ..266/3 R;148/134, Zone before full temperature q z n, h ro 148/145, 156 beingreeled when a substantial peripheral zone of the rod has beenpredominantly transformed to pearlite.

3 Claims, 2 Drawing Figures t(sek) Patented May 29, 1973 3,735,966

2 Sheets-Sheet l r I v *1 0,01 0, 1,0 10 100 1000 t(sek) Fly.

INVENTOR.

BERND HOFFMANN Patented May 29, 1973 2 Sheets-Sheet 2 INVENTOR.

BERND HOFFMANN METHOD FOR HEAT TREATING STEEL WIRE ROD This applicationis a continuation of Ser. No. 743,101, filed July 8, 1968, and nowabandoned.

The invention relates to an apparatus for heat treating of hot,patentable steel rod (for instance rod having a carbon content of moreor slightly more than 0.4 percent by weight) from the rollingtemperature, in which after the rod leaves the last stand of a rod mill,the surface is quenched in successive stages and reheated by temperaturecompensation with its internal crosssection, and subsequentlytransformed in the pearlite transformation range.

According to the present invention, there is provided an apparatus forheat treating hot, patentable steel rod after the rod leaves the laststand of a rod mill, comprising means for quenching the surface of therod to a temperature of about 70C above the martensite transformationtemperature in each of a number of successive quenching stages andallowing the surface of the rod to be reheated after each quenchingstage by heat from the center zone of the rod so that the rod enters thepearlite transformation zone when temperature compensation is stilloccurring between the center and peripheral zones of the rod and with anaverage temperature of about 600 to about 650C and the temperature ofthe rod surface is raised to above the upper bainite temperature beforetransformation begins, and reeling the rod when a substantial peripheralzone of the rod is predominantly transformed to a pearlitic structure.

Preferably, said peripheral zone has a radial thickness of about toabout 20 percent of the rod diameter. Hot rolled rods usually have adiameter of between 5 and 12.7 millimeter, and in the case of a thickrod,

said peripheral zone may have a radial thickness of I about 10 percentof the rod diameter whereas in the case of a thin rod, said peripheralzone may have a radial thickness of about 20 percent of the roddiameter. Preferably, the rod is reeled when the peripheral zone hasbeen 50 to 70 percent transformed to a pearlitic structure.

Once the transformation of the peripheral zone has occurred in .themanner described, the remaining heat content of the center zone is notable to transmit a high enough degree of heat to the peripheral zone asto cause the pearlite to be transformed back into austenite, which wouldinvolve the risk of renewed martensite formation. On the other hand, theperipheral zone prevents the spontaneous cooling of the center zone,

tact between the surface of the rod and the cold portions of the reel(or conveyor); such spontaneous cooling'would result in the formation ofmartensite in the core.

Additional protection against too much heat being withdrawn from themetal when the surface comes into contact with the cold portions of thereel is afforded by the thin layer of scale which forms on the surfaceof the rod during cooling. This offers substantial resistance to theconduction of heat and thus also favors a transformation process whichavoids the formation of martensite or bainite.

The method performed by the apparatus of the invention whereby thesurface of the rod is cooled in intermittent stages down 400C or lesscan be used directly after the rolled rod leaves the finishing stand oft the rod mill. Due to the strong cooling effect which has to beemployed, the first stage of quenching produces a structure which isexceedingly fine-grained because of the foregoing deformation. Thisresults in a shift of the time-temperature transformation curves for thebeginning and end of the pearlite transformation in such a way thatpearlite transformation begins and finishes very early. This means thatit is possible considerably to shorten the cooling system which followsdirectly after the finishing stand and to make it of a lengthcorresponding to the product of the rolling speed and a time of about0.6 to about 0.7 seconds. When, for example, the rolling speed is 50m/s, the cooling system only need be between 30 and 42 meters in length.Once the peripheral zone of the rod has entered the zone of pearlitetransformation, the rod may be laid in nonconcentric turns or coiledwithout any risk of the local formation of undesirable structures in theperipheral zone. The contact between the surface of the rod and thecooler portions of the reel thus no longer has any effect and insteadalmost the whole of the resulting structure is sorbitic.

In the cooling system, which may operate by the alternate effects ofwater and air, there is now no formation of martensite, in spite of thefact that the surface temperature of the rod is near that of martensitetransformation. This is due to the fact that the center zone of the rodstill contains a sufficient amount of heat that it is able to reheat thesurface before the next watercooling stage to such a degree that evenduring water cooling the temperature never drops below the point ofmartensite transformation. Once the average crosssectional temperatureof the wire rod has reached 600 650C, pearlite transformation commencesin the peripheral zone. The intermittent cooling to below 400C and to atemperature of about C above the martensite transformation temperatureresults not only in the cooling time being substantially shortened butalso in the fact that, as a result of the shift in the pearlitetransformation range, conditions favor optimum cooling during thisshorter period.

The apparatus comprises a series of alternate water and air coolingunits through which the rod is arranged to pass, for positioning afterthe last stand of a rod mill but before a reel, the cooling effect ofthe water cooling units becoming less (for instance the water coolingunits becoming shorter) in the direction of travel of the rod accordingto a logarithmic relationship and the aircooling units becoming longerin the direction of travel of the rod according to a logarithmicrelationship, the overall length of the cooling units being equal to theproduct of the rod speed and a time of about 0.6 to about 0.7 seconds.According to the present invention, there is in addition provided acombination of a rod mill, apparatus according to the invention and areel, for rolling, heat treating and reeling hot, patentable steel rod.

The invention will be further described, byway of example, withreference to the accompanying drawings, in which:

FIG. 1 is a time-temperature transformation diagram; and

FIG. 2 is a schematic elevation of apparatus in accordance with theinvention.

FIG. 1 could illustrate the transformation of any of the steels EN 42and EN 42 B to EN 42 1 (British quality); EN 42 B corresponds to DIN11221 (German standard). FIG. 1 shows, in the usual manner, thetransformation curves and the transformation temperatures A A, and M,Pearlite transformation takes place in zone I, ferrite transformation inzone II and bainite (intermediate stage) transformation in zone III.Pearlite transformation commences at a temperature of about 600 afterabout 0.6 seconds, due to the fine-grain formation of the structureobtained as a result of the strong cooling effect. The rod leaves thefinishing stand at a temperature of about 950C. Three temperature curvesillustrate what occurs during the cooling process effected directly therod leaves the mill. Curve a represents the temperature of the rod core,curve b the average temperature of the rod as a whole and curve thesurface temperature. For reasons of clarity only three successivequenching processes are illustrated. Generally a greater number ofsuccessive cooling processes will be possible, more quenching operationsbeing feasible in the case of small diameter rods than in that oflarge-diameter rods, because of the more rapid equalization oftemperature between the core and the periphery. At each quenching stagethe metal is cooled to temperature T,, which, as already mentioned, isbelow 400C and is about 70C above the martensite transformationtemperature M,,. During the following air-cooling stage, the effect ofwhich is much weaker, the surface temperature rises due to the flow ofheat from the center to the surface of the rod, and is then recooled. Acomplete equalization of temperature between the core and periphery doesnot occur since this would require more time. The temperature of thecore, as illustrated by curve a and the average temperature asillustrated by curve b have an approximately logarithmic shape which isslightly wavy in accordance with the varying conditions of cooling atthe surface of the rod. The last cooling stage in the system may haveits effect shortly before pearlite transformation commences sincetransformation does not require that the temperature of the core, theaverage temperature of the rod as a whole, and the surface temperatureshould closely approximate. It is sufficient that the surfacetemperature be, 'as shown in the drawing, increased (by the process oftemperature equalization which results from the higher temperature ofthe core) just to the point which ensures that the metal does not passthrough the bainitic transformation zone Ill. It is, of course, possiblealso to advance the last quenching stage so that the three temperaturesdevelop almost uniformly or reach almost the same value before enteringthe lower pearlite zone.

After pearlite transformation has commenced in zone I and a peripheralzone of the rod equal to to percent of the rod diameter has undergone a50 70 percent transformation, it is possible to lay the wire rod inloops or coil or reel it, as already described, without incurring therisk of undercooling causing martensite or bainite formation in theperipheral zone. Thorough tests, during which portions of rod werequenched immediately laying or coiling commenced in order to effecttransformation of the residual austenite, confirmed that the peripheralzone was only composed of pearlite, with the possibility of someferrite. The more intensive cooling of the peripheral zone made itpossible to achieve, in addition to a fine-grain structure, aconsiderable increase in the buckling resistance of the rod, thusimproving guidance of the rod.

in order to achieve the cooling effect desired, it is sufficient andalso advisable that the surface of the rod be reheated after eachwater-cooling stage to a temperature which is below 600C and that thefollowing quenching process commences at this temperature.

As already mentioned, the cooling system comprises alternating water andair cooling units. FIG. 2 shows this cooling system, which follows a rodmill finishing stand having rolls 1 and 2. The layout was designed onthe basis of a final rolling speed of 40 meters/sec. and an overallthroughput time of 0.6 sec. The total length of the cooling system is24m.

The rolling stand is directly followed by a cooling tube 3 from whichwater is conveyed on to the rolled material W, shown as a broken line;this is followed by an air-cooling unit 4. This in turn is followed byanother water-cooling unit 5 succeeded by an air-cooling unit 6. If weassume that the cooling effect in the individual units is proportionalto the length shown in the diagram we arrive at the fact that, in viewof the decreasing temperatures shown in FIG. 1, the length of thecooling system units shown in the drawing at the odd numbers steadilydecreases as the rod passes through the cooling system. The individualwater-cooling units 3,5,7,9 and 11 are of logarithmically relatedlengths. Conversely, in view of the gradual reduction in the heatcontent of the rod, the time required for reheating increases a factwhich has been taken into account from the point of view of the coolingsystem in that these air-cooling units 4,6,8,10 are of logarithmicallyincreasing length. The final air-cooling unit 12 is, on the other hand,somewhat shorter in order to compensate for the extra air cooling whichstill takes place in the laying reel 13.

In the reel 13, the rod W is shifted out of its horizontal path so as toform loops 14 which tip forwards over an edge surface (not shown indetail in the drawing) and out of the reel so as to tilt and dropdownwards, as illustrated by loop 15. They are then conveyed forward bythe belt 17 in successive and partly over-lapping loops 16, to which endthe conveyor belt 17 is passed over rollers 18, at least one of which isdriven.

I claim:

1. Apparatus for heat treating hot, patentable steel rod whosetemperature is initially above the A transformation point, as the rodissues from the last stand of a rod mill, the apparatus comprising aseries of at least three successive water cooling units and at leastthree successive air cooling units arranged as alternate water and aircooling units through which the rod is adapted to pass, the coolingeffect of the water cooling units becoming less in the direction oftravel of the rod according to a logarithmic relationship and the aircooling units becoming longer in the direction of travel of the rodaccording to a logarithmic relationship, the overall length of thecooling units being equal to the product of the rod speed and a time ofabout 0.6 to about 0.7 seconds.

2. Apparatus as claimed in claim 1, wherein the water cooling unitsbecome shorter in the direction of travel of the rod according to alogarithmic relationship.

3. Apparatus as claimed in claim 1 in combination with a reel, forrolling, heat treating and reeling hot,

patentable steel rod.

t: k k 4i

1. Apparatus for heat treating hot, patentable steel rod whosetemperature is initially above the A3 transformation point, as the rodissues from the last stand of a rod mill, the apparatus comprising aseries of at least three successive water cooling units and at leastthree successive air cooling units arranged as alternate water and aircooling units through which the rod is adapted to pass, the coolingeffect of the water cooling units becoming less in the direction oftravel of the rod according to a logarithmic relationship and the aircooling units becoming longer in the direction of travel of the rodaccording to a logarithmic relationship, the overall length of thecooling units being equal to the product of the rod speed and a time ofabout 0.6 to about 0.7 seconds.
 2. Apparatus as claimed in claim 1,wherein the water cooling units become shorter in the direction oftravel of the rod according to a logarithmic relationship.
 3. Apparatusas claimed in claim 1 in combination with a reel, for rolling, heattreating and reeling hot, patentable steel rod.